Genes coding for the specialized products of differentiated mammalian cells have in general been difficult to map. To date none of the genes specifying enzymes involved in neurotransmitter metabolism have been assigned to specific chromosomes. We have a cell system that is anticipated to provide the means for mapping the human structural gene for tyrosine hydroxylase, the presumed rate-limiting enzyme for the biosynthesis of the catecholamine neurotransmitters dopamine and norepinephrine. Human neuroblastoma cells expressing high levels of tyrosine hydroxylase activity have been fused with mouse neuroblastoma cells that do not express this activity. Hybrid clones, selected with use of a positive selection system for the gene of interest (based on ability of cells with tyrosine hydroxylase activity to grow in medium devoid of the essential amino acid tyrosine), have measurable levels of the hydroxylating enzyme. Initial Giemsa-banding karyotype analyses showed preferential segregation of human, not mouse, chromosomes. Currently available human-mouse neuroblastoma cell hybrids and planned hybridizations involving human neuroblastoma clones are expected to enable us to: (1) assign a human chromosomal location for the tyrosine hydroxylase gene; (2) determine whether the long, homogeneously staining region, HSR, a unique metaphase chromosome abnormality characterizing many continuously cultured human neuroblastoma lines, may be associated with expression of the neuronal phenotype; (3) determine whether expression of L-amino acid decarboxylase of dopamie-beta-hydroxylase activity is correlated with expression of tyrosine hydroxylase. In addition to new knowledge of gene mapping and gene expression, the proposed study may provide new insights into regulation of catecholamine biosynthesis in these human tumor cells.